Oracle : SQL replace a value in a column if override exists

Question

I have table like below :

PRODUCT     COLLECTION_DATE      DATA      OVERRIDE
-------     ---------------      ----      --------
Laptop      2014/11/01           62          
Laptop      2014/12/01           62        75
Laptop      2015/01/01           62        
Laptop      2015/02/01           62        
Laptop      2015/03/01           62        83
Laptop      2015/04/01           62          
Laptop      2015/05/01           62
Laptop      2015/06/01           62

I need to override the values in DATA column with the values from OVERRIDE column to produce below data

PRODUCT     COLLECTION_DATE      DATA      OVERRIDE
-------     ---------------      ----      --------
Laptop      2014/11/01           62          
Laptop      2014/12/01           75        75
Laptop      2015/01/01           75        
Laptop      2015/02/01           75        
Laptop      2015/03/01           83        83
Laptop      2015/04/01           83          
Laptop      2015/05/01           83
Laptop      2015/06/01           83

How can I do that in SQL? Thank you in advance.

Answer 1

You can do this in a single row using coalesce() :

select product, collection_date, coalesce(override, data) as data, override
from table t;

You seem to want the most recent non-NULL value. In Oracle 11+, you can use the ignore null option on lag() :

select product, collection_date, 
       coalesce(lag(override ignore null) over (partition by product order by collection_date), data) as data,
       override
from table t;

If you are using an earlier version of Oracle, and override is always positive, you can take the cumulative sum to define groups with the same value. Then use this for the value:

select product, collection_date,
       coalesce(max(override) over (partition by product, grp order by collection_date), data) as data,
       override
from (select t.*, sum(override) over (partition by product order by collection_date) as grp
      from table t
     ) t;